overload Preferences.getBytes similar to nvs so you can get size of t… (espressif#2498)

* overload Preferences.getBytes similar to nvs so you can get size of the array. * Cleaner implentation, with a separate function to get length. Added an example

Author: lbernstone

libraries/Preferences/examples/Prefs2Struct/Prefs2Struct.ino

@@ -0,0 +1,43 @@
+/*
+This example shows how to use Preferences (nvs) to store a
+structure. Note that the maximum size of a putBytes is 496K
+or 97% of the nvs partition size. nvs has signifcant overhead,
+so should not be used for data that will change often.
+*/ 
+#include <Preferences.h>
+Preferences prefs;
+
+typedef struct {
+ uint8_t hour;
+ uint8_t minute;
+ uint8_t setting1;
+ uint8_t setting2;
+} schedule_t;
+
+void setup() {
+ Serial.begin(115200);
+ prefs.begin("schedule"); // use "schedule" namespace
+ uint8_t content[] = {9, 30, 235, 255, 20, 15, 0, 1}; // two entries
+ prefs.putBytes("schedule", content, sizeof(content));
+ size_t schLen = prefs.getBytesLength("schedule");
+ char buffer[schLen]; // prepare a buffer for the data
+ prefs.getBytes("schedule", buffer, schLen);
+ if (schLen % sizeof(schedule_t)) { // simple check that data fits
+ log_e("Data is not correct size!");
+ return;
+ }
+ schedule_t *schedule = (schedule_t *) buffer; // cast the bytes into a struct ptr
+ Serial.printf("%02d:%02d %d/%d\n", 
+ schedule[1].hour, schedule[1].minute,
+ schedule[1].setting1, schedule[1].setting2);
+ schedule[2] = {8, 30, 20, 21}; // add a third entry (unsafely)
+// force the struct array into a byte array
+ prefs.putBytes("schedule", schedule, 3*sizeof(schedule_t)); 
+ schLen = prefs.getBytesLength("schedule");
+ char buffer2[schLen];
+ prefs.getBytes("schedule", buffer2, schLen);
+ for (int x=0; x<schLen; x++) Serial.printf("%02X ", buffer[x]);
+ Serial.println(); 
+}
+
+void loop() {}

libraries/Preferences/src/Preferences.cpp

@@ -447,21 +447,29 @@ String Preferences::getString(const char* key, const String defaultValue){
  return String(buf);
 }
 
-size_t Preferences::getBytes(const char* key, void * buf, size_t maxLen){
+size_t Preferences::getBytesLength(const char* key){
  size_t len = 0;
- if(!_started || !key || !buf || !maxLen){
+ if(!_started || !key){
  return 0;
  }
  esp_err_t err = nvs_get_blob(_handle, key, NULL, &len);
  if(err){
  log_e("nvs_get_blob len fail: %s %s", key, nvs_error(err));
  return 0;
  }
+ return len;
+}
+
+size_t Preferences::getBytes(const char* key, void * buf, size_t maxLen){
+ size_t len = getBytesLength(key);
+ if(!len || !buf || !maxLen){
+ return len;
+ }
  if(len > maxLen){
  log_e("not enough space in buffer: %u < %u", maxLen, len);
  return 0;
  }
- err = nvs_get_blob(_handle, key, buf, &len);
+ esp_err_t err = nvs_get_blob(_handle, key, buf, &len);
  if(err){
  log_e("nvs_get_blob fail: %s %s", key, nvs_error(err));
  return 0;

libraries/Preferences/src/Preferences.h

@@ -63,6 +63,7 @@ class Preferences {
  bool getBool(const char* key, bool defaultValue = false);
  size_t getString(const char* key, char* value, size_t maxLen);
  String getString(const char* key, String defaultValue = String());
+size_t getBytesLength(const char* key);
  size_t getBytes(const char* key, void * buf, size_t maxLen);
  size_t freeEntries();
 };